I wasn't aware that people in the US don't want to talk about 9/11. There have been multiple books and movies about the failure to prevent the terrorist attack, the attack itself, and years of aftermath. Public discourse about the event isn't suppressed, and even George W Bush has commented about the event while he was president and after he left office.
Though I've seen it referenced multiple times, I don't see the connection between Hong Kong's protests and 9/11.